Technical Abstract:
Recombinant E. coli strains have been developed for the conversion of glucose as well as pentose sugars into L-lactic acid. The strains carry the lactate dehydrogenase gene (ldh) from Streptococcus bovis on a plasmid. Because the untransformed strains have a genetic background (ldhA and pfl) that renders them incapable of fermentative growth, the transformed strains sselectively maintain the plasmid when cultured anaerobically. Cultures of E. coli B strains expressing ldh from either a low or high copy number plasmid were determined to have similar lactic acid yields (93-95% of theoretical) when used to ferment 10% w/v glucose. A medium optimization study was subsequently undertaken to develop a formulation with fewer complex components, which would be less expensive and more practical for recovery of lactic acid. The strains selected for glucose fermentations were also tested on glucose and xylose mixtures. Fermentations stalled after completion of glucose and before utilization of xylose. A new lacti acid producing strain was subsequently constructed that carries a glucose phosphotransferase mutation (ptsG), which allows for simultaneous utilization of glucose and xylose. Presence of the ptsG mutation improved the final lactic acid yield for the sugar mixture by 54% compared to the non-ptsG control strain because of improved fermentation of the xylose.